Syntheses of a benzoxazine resin have been reported in Journal of Organic Chemistry (J. Org. Chem), vol. 30, p. 3423 (1965) (author; Burke), Japanese Provisional Patent Publications No. 155234/1985, No. 177074/1985 and No. 47378/1974, and Journal of Polymer Science Part A: Polymer Chemistry (J. Polym. Sci. Part A: Polym. Chem.), vol. 32, p. 1121 (1994) (authors; Ishida et al.). According to these reports, as a synthesis method of a benzoxazine resin, there are disclosed that (1) a method in which formaldehyde is added to a mixed solution of a compound having a phenolic hydroxyl group and a primary amine, (2) a method in which a phenolic hydroxyl group is added to a solution obtained by reacting a primary amine with formaldehyde and (3) a method in which it is synthesized by a method of adding a mixture of a primary amine and a phenolic hydroxyl group to formaldehyde, and then, removing a reaction solvent and condensation water produced at the time of synthesis under a reduced pressure.
When the formed condensation water and the reaction solvent are removed under a significantly reduced pressure from the start, the temperature of a reaction solution abruptly drops due to heat deprived by volatilization of the water and reaction solvent, whereby the temperature of the reaction solution becomes much lower than the softening point of a synthesized resin. In this case, since a benzoxazine resin has neither water solubility nor hydrophilicity, the viscosity of the reaction solution increases, so that stirring of the reaction solution becomes impossible and it takes enormous efforts to discard it.
The present invention has been done to solve these problems. A first object of the present invention is to produce a benzoxazine resin safely. Also, a second object of the present invention is to remove generated condensation water and an organic solvent with good efficiency. Moreover, a third object of the present invention is to adjust the molecular weight of an obtained benzoxazine resin with good efficiency.
As a result of earnest studies and developments by the present inventors, these problems of the present invention have been overcome based on the novel findings mentioned below.